<!DOCTYPE html>
<html>

<head>
    <meta http-equiv="content-type" content="text/html; charset=utf-8">
    <meta name="viewport" content="width=device-width,initial-scale=1,maximum-scale=1,user-scalable=no">

    <title>伸缩版搜索框</title>
    <link href="https://cdn.bootcdn.net/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">
    <style>
        body {
            /* 初始化 取消内外边距 */
            margin: 0;
            padding: 0;
            /* 100%窗口高度 */
            height: 100vh;
            /* 渐变背景 */
            background: linear-gradient(200deg, #e3eeff, #f3e7e9);
        }
        
        .search-box {
            /* 绝对定位 水平垂直居中 */
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
            background-color: #2f3640;
            height: 40px;
            padding: 10px;
            border-radius: 40px;
        }
        
        .search-txt {
            border: none;
            background: none;
            outline: none;
            float: left;
            padding: 0;
            color: #fff;
            font-size: 16px;
            line-height: 40px;
            width: 0;
            /* 动画过渡 */
            transition: 0.4s;
        }
        
        .search-btn {
            color: #e84118;
            float: right;
            width: 40px;
            height: 40px;
            border-radius: 50%;
            background-color: #2f3640;
            /* 弹性布局 水平垂直居中 */
            display: flex;
            justify-content: center;
            align-items: center;
            cursor: pointer;
            /* 动画过渡 */
            transition: 0.4s;
        }
        
        .search-box:hover .search-txt {
            width: 200px;
            padding: 0 6px;
        }
        
        .search-box:hover .search-btn {
            background-color: #fff;
        }
    </style>
</head>

<body>
    <div class="search-box">
        <input type="text" class="search-txt" placeholder="想搜啥?" />
        <a class="search-btn">
            <i class="fa fa-search" aria-hidden="true"></i>
        </a>
    </div>
</body>

</html>